This invention relates to a method of estimating geo-stress or ground pressure making use of the Kaiser effect of acoustic emission and, more particularly, a method of estimating ground pressure by applying load to each rock sample extracted from the ground, counting the number of emissions of sound during the application of the load and determining the ground pressure from the magnitude and direction of residual stress in the ground where the rock sample was extracted.
When removing rocky ground, cutting a tunnel, cutting a level or a shaft for extracting underground resources, or building a large-scale underground structure for an atomic power station or the like, it is important from the standpoint of the work plan to grasp the stress state of the relevant ground beforehand. The stability of the ground after the work is completed is greatly influenced by the magnitude and direction of the ground pressure, and there are many cases leading to, for instance, collapse of excavated surface, shift or collapse of tunnel side walls, rock burst and so forth.
There are several conventional methods of measuring the actual geo-stress, the ground pressure state under the prevailing condition of the ground. In any of these methods, however, some gauge or measuring instrument is buried in the ground for measurement, thus requiring considerable time and expense in addition to the fact that it is difficult to obtain a sufficiently great amount of data. The so-called over-coring method is a typical conventional method. In the over-coring method, a strain gauge is buried in a bore of a small diameter (of about 60 mm), and thereafter a concentric over-coring bore of a large diameter (of about 200 mm) is bored for estimating the ground pressure by measuring the strain in the state with stress released. This method requires a period of about a month and the afore-mentioned large-diameter boring for each point of measurement. Therefore, it sometimes happens that measurement is impossible or it is difficult to obtain many samples depending upon the site.
Meanwhile, a phenomenon commonly termed Kaiser effect is known in the acoustic emission test method. The Kaiser effect occurs when that the material structure is steady and stable and is less subject to acoustic emission up to the prior hysteretic maximum stress, but the structure becomes liable to microscopic destruction due to increased acoustic emission when it receives a stress in excess of the aforementioned stress. This effect is utilized for testing the presence or absence of the reduction of the structural quality of metal structures, such as pressure containers or pipe lines, during their period of operation, but in the field of testing of metallic materials the directivity of the Kaiser effect is not yet clear yet.
This invention is predicated in the finding that rocks also present the Kaiser effect and that directivity is present in that effect.
Accordingly, an object of the present invention is to provide a method of estimating ground pressure, which allows a simple, ready and economic estimation of the ground pressure.
Another object of the present invention is to provide a method of estimating ground pressure, without having to bury a strain gauge or like instrument in the ground.